1. Field of the Invention
This invention generally relates to digital communications and, more particularly, to a system and method for increasing throughput and reducing latency in a Multimedia over Coax Alliance (MoCA) network of connected devices.
2. Description of the Related Art
Consumers are migrating to increasingly complex entertainment technologies such as digital video recorders (DVRs) and high-definition television (HDTV) content, delivered via cable and satellite. The popularity of time-shifted viewing, as permitted by a VCR, is creating a demand for multi-room solutions that enable recorded content to be served from a common hard-drive based DVR and watched in any room in the house. However, the primary driver behind operator-installed entertainment networks is the transition from single HDTV households to multiple HDTVs per home.
The absence of an entertainment network currently requires operators to employ analog distribution to enable whole-home DVR functionality. Analog video distribution can provide a relatively simple solution to whole-home DVR and allows content to be accessed by other TVs in the home without requiring separate decoding of the digital video signal. However, this solution severely limits quality and functionality and may require separate coax runs and switches making it an unattractive solution. Distributing recorded high-definition content to secondary TVs within the home requires the content to be down-converted to standard-definition and distributed over the existing coax wiring to other locations within the home.
Despite the many advantages of coaxial cabling as a home networking medium, transmitting packets at high speed over coax does present challenges, as signals transmitted over coax can generate echoes and attenuation. These problems are addressed by the MoCA standard. Cable and satellite operators both carry services over coaxial cabling, but each operates in unique segments of RF spectrum. Home networking technologies operating above 860 MHz are ideally suited to coexist with cable service offerings as they occupy space above both the downstream frequencies used to carry video content and baseband frequencies which carry upstream data traffic from Data Over Cable Service interface Specification (DOCSIS) cable modems. The MoCA spectrum of frequencies also avoids satellite Internet broadband data connectivity through third party operators in the 18-40 gigahertz (GHz) Ka-Band, and the L-Band spectrum used for video services. MoCA's networking technology operates in the open band of spectrum between 860-950 megahertz (MHz).
MoCA nodes are typically able to communicate at data rates of approximately 100 megabits per second (Mbps). A 100 Mbps net data rate provides enough headroom to enable multiple HD and SD streams at peak data rates to coexist on the same network along with voice and data traffic. In addition, a guaranteed 100 Mbps net data rate enables multiple HD and SD streams of “trick-mode” (e.g., fast-forward or reverse) viewing from a central DVR under all but the most extreme circumstances. However, as technologies evolve, a demand for additional bandwidth typically occurs.
As with most other communication systems, MoCA devices are organized in a suite of layered protocols, where a media access control (MAC) layer interfaces between the physical layer (PHY) and upper layers. As conventionally configured, there are limitations in the MoCA MAC throughput. In a MoCA network, the maximum MAC rate is achieved when the user packets are the same size as the maximum transmission unit (MTU) size, which at the time of this writing is 1518 bytes. For smaller size user packets, the effective MoCA MAC throughput drops due to the following reasons:
1. PHY Packet Overhead: the transmission of each PHY packet is pre-pended with a fixed-size PHY Preamble;
2. Overall per-packet protocol overhead. This metric includes the bandwidth request and grant protocol overhead for each packet; and,
3. An additional requirement on the minimum time between the starts of two consecutive packets. The current MoCA standard requires that the minimum transmission time allocated to a packet transmission is to be 52 us. If a request is made for a short packet with a transmission time of less than 52 us, the transmission is followed by an idle time to meet the minimum packet transmission time. The effective MAC rate drops to 10 Mbps for 64-byte packets and 40 Mbps for 256-byte packets, as compared to 135 Mbps for MTU sized (1518-byte) packets.
There is also a latency issue inherent in the MoCA protocols. In a MoCA network, the transmission of a client data packet is store-and-forward. This technique requires the transmitting node to wait for a complete user packet to be received, before a request can be sent and grant received. This process introduces a minimum latency of one media access plan (MAP) cycle, and an average latency of two MAP cycles before the transmission of a user packet. The nominal MAP cycle is 1 ms, and the minimum MAP cycle is greater than 400 us.
It would be advantageous if medium transmission utilization could be improved in a MoCA network and the effective MAC throughput increased for small size packets. It would also be advantageous if the latency through the MoCA network could be reduced.